TORONTO  In a discovery with implications for fighting obesity in humans, Canadian scientists have discovered a molecular switch in specially bred laboratory mice that makes the animals skinnier than their normal brethren.

The altered mice, which lack a certain gene, have half as much fat as normal mice -- and the fat they do have isn't the kind that piles on the weight, say researchers at the Ottawa Health Research Institute.

Anthony Scime, a molecular biologist at the University of Ottawa institute, said the leaner mice have a higher proportion of what are known as brown fat cells, which burn up fat and release it as heat.

Normal mice -- and humans and other mammals, for that matter -- have mostly white fat cells, which metabolize fat as energy to fuel muscles and other bodily functions.

So when food intake exceeds energy output, these white fat cells multiply and expand in girth, Scime said Tuesday from Ottawa.

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"You just keep on getting bigger and bigger," he said.

The knocked-out gene in the mice -- called P107 -- seems to act as a switch on precursor cells, causing them to develop into heat-producing brown fat cells instead of lipid-storing white fat cells, said lead investigator Michael Rudnicki, director of molecular medicine at the institute.

"The P107 knockout mice eat just as much as normal mice, but they burn off all the extra calories (as heat)," Rudnicki said.